Control system for mask etching apparatus

ABSTRACT

A mask etching apparatus has a succession of etching stations to which etchant is supplied from a common source through a closed circulating distribution system. Measuring devices monitor the etching process by measuring the hole size of the mask in process and develop a control potential through which the duration of the etching time is controlled in the carious stations. When no etching is to take place in a given station, a valve to that station is closed but a by-pass permitting a restricted flow of etchant continues circulation through the entire system.

Nov. 7, 1972 M. LERNER 3,702,277

CONTROL SYSTEM FOR MASK ETCHING APPARATUS Filed Sept. 25, 1970 Programmer 8 Conveyor 35 55 35 35 Oxide Sdripper on Rinse l J v 1 l fi I Rinse IO I 1st. 2nd. 3rd. 4th De z' l STOTIOH.STOTIOH Station STOTIOHT Q U [L U 2 Di. Rinse l4 {r w Etchont 25 Source Densitomete ensitometer Control Control Programmer inventor Martin L. Lerner Attorney United States Patent 3,702,277 CONTROL SYSTEM FOR MASK ETCHING APPARATUS Martin L. Lerner, River Forest, 11]., assignor to Zenith Radio Corporation, Chicago, Ill. Filed Sept. 23, 1970, Ser. No. 74,760 Int. Cl. C23f 17/00; C03c 15/00; B23p 15/00 US. Cl. 156-345 3 Claims ABSTRACT OF THE DISCLOSURE A mask etching apparatus has a succession of etching stations to which etchant is supplied from a common source through a closed circulating distribution system. Measuring devices monitor the etching process by measuring the hole size of the mask in process and develop a control potential through which the duration of the etching time is controlled in the various stations. When no etching is to take place in a given station, a valve to that station is closed but a by-pass permitting a restricted flow of etchant continues circulation through the entire system.

CROSS-REFERENCES TO RELATED APPLICATIONS The subject application is a further development of the etching apparatus and process described in application Ser. No. 6,619 filed Jan. 28, 1970 and application Ser. No. 45,681 filed June 12, 1970, both in the name of Martin Lerner. All such applications are assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION The present invention is concerned with an improved apparatus and system for etching the shadow mask components of color cathode-ray tubes. It is generally useful in providing such masks with a pattern of apertures or holes of a predetermined size but is particularly advantageous in connection with re-etch systems of mask processing.

Re-etching is a technique that has been adopted in the manufacture of color picture tubes featuring black-surround screens of the type described and claimed in Pat. No. 3,146,368, issued Aug. 24, 1964, in the names of Joseph P. Flore et al. Such a screen in its preferred form is a mosaic structure comprised of a multiplicity of phosphor dot triads disposed in an ordered array throughout the image or screen area. Each such triad includes a dot of green phosphor, a dot of blue and a dot of red phosphor. The screen differs from conventional mosaic color screens by having phosphor dots reduced in dimension so that, instead of being in tangential contact with one another, they are separated from one another. Another particularly important and distinguishing feature is that the spaces which surround the phosphor dots are covered with a light-absorbing material, such as graphite. In short, each phosphor dot has a circumscribing ring of dark, light-absorbing material from which the screen derives its name black-surround.

It is common practice in screening such a tube to locate and dimension deposits of the various phosphor materials by photographic printing in which a photosensitive material is exposed to actinic energy directed from a source to the screen through the holes of a shadow mask assigned to that screen and positioned in operating relation thereto during the exposure step. In order to have phosphor dots of reduced size, while retaining the efiiciency and tolerance characteristic of conventional triad color tubes, it is distinctly preferred that the electron beams of tube be larger in diameter than the phosphor dots. This obviously presents problems in screening, namely, the problem of establishing desired relative dimensions of phosphor dots to excitation electron beams.

A most successful process of screening, achieving the desired relative dimensions, features the use of a shadow mask having a pattern of holes initially dimensioned as required to screen the phosphor materials on the tube faceplate. After screening has been accomplished through well-known photographic techniques, the mask is subjected to further etching to enlarge the holes to a precisely controlled size in order that electron beams, which obviously are dimensioned by the holes of the mask through 'which the beams reach the screen, have a larger, and accurately determined diameter, than the phosphor dots. This general process is referred to as etch-back or re-etch. A control system for precisely dimensioning the mask apertures in the re-etch process is the subject of Lerner application No. 6,619. The other Lerner application discloses a preferred orientation of the mask in process, with particular regard to the location of the spray heads through which an etchant is directed to the mask during re-etch.

It is an object of the present invention to further improve the etching of holes in the shadow mask to attain a predetermined and precisely controlled size.

It is another and particular object of the invention to improve the etch-back system of shadow mask processing.

SUMMARY OF THE INVENTION A system embodying the invention for etching holes in shadow masks to a predetermined size comprises a plurality of etch stations, individually haying spray means for directing an etchant vertically upward to contact and etch a mask instantaneously located within the particular station. There are means for conveying a mask through the various stations, seriatim, with a dwell time of given duration in each such station. A fluid distribution system, including a source of etchant, maintains a flow of etchant between the source and the spray means of the various stations. The valves in the distribution system in series with the spray means are shunted by a bypass for admitting, during intervals when an associated valve is closed, a restricted flow of etchant to the spray means in series with that valve to maintain a flow of etchant but in an insufficient amount to etch a mask. Finally, a system measures the hole size of a mask after it has been etched in one of the stations to derive a control potential utilized to selectively control the valves and the etching of the mask in subsequent stations while retaining a restrictive flow of etchant to the spraying means in any of the stations in which the main valve has been closed.

BRIEF DESCRIPTION OF THE DRAWING The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description in connection with the accompanying drawing and in the several figures of which like reference numerals indicate identical elements and in which:

'FIG. 1 represents an etching system embodying the invention; and

FIG. 2 is a view showing details of the control associated with each of the etching stations of the system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, the system of FIG. 1 is for etching shadow masks or, as they are sometimes referred to, color-selection electrodes of the shadow mask type of color picture tube. Although the system is useful in establishing a pattern of apertures in blank sheets of metal stock, it is of particular benefit in practicing re-etch in the screening of black-surround tubes and will be described in that connection simply as a matter of convenience.

The system is arranged for mass production in which a conveyor 8 presents a succession of shadow masks seriatim to the various work stages of the apparatus. The first stage 10 to which the conveyor presents a mask is an oxide stripper which is employed to determine the size and location of phosphor dots. It is common practice to blacken the mask or provide it with an oxide coating so that its radiation properties are analogous to those of a black body. The oxide, however, resists the attack of an etchant that otherwise would be eifective in enlarging the holes of the mask in process. The mask usually is formed of cold rolled steel and a suitable etchant is ferric chloride. In order to avoid attenuating the effect of the etchant, the mask is treated at work station 10 with hydrochloric acid and an inhibitor which strip the oxide coating after which the mask is rinsed with water and conveyed to the following stage.

The next stage 11 is a four-station etching arrangement and each mask is advanced, step-by-step, through the various stations by means of conveyor 8 and provided with a dwell time of about 10 to 12 seconds per station. The dwell or process time available at each etching station is determined in the usual manner by means of a conventional programmer or timer 15 which controls the step-by-step advance of the conveyor as required to present each mask to the various etch stations and also energizes electrically actuated valves to control the etch process in properly timed relation to the travel of the shadow masks. It is convenient from the standpoint of uniformity and process control to have each of the etching stages fed from a common etchant source or supply 12 so that such parameters as concentration and temperature of the etchant are the same in each station. Source 12 is included in a closed fluid distribution system which sustains flow of etchant between the source and the various etching stations as indicated by the supply manifold 13 and the exhaust or return manitold 14.

Certain details of the etchant distribution system appear in FIG. 2 which is a view of one etching station, indicating that each such station has a spray means for directing the etchant vertically upward to contact and etch a mask 9 instantaneously within the particular station. It has been found particularly advantageous to have spray means 20 comprise a series or array of nozzles 21 distributed along a conduit 22 in communication with input manifold 13 so that etchant passing through the spray head is directed in the form of an elongated narrow ribbon against the adjacent surface of mask 9. In order to distribute the etchant uniformly over the entire surface of the mask, the array of nozzles is rotatable as indicated by the pulley 23. A catch pan 24 or the like reclaims the etchant that falls back from the mask and delivers it to the return conduit 14. An electrically actuated main valve 25 is provided for each station in series with its spray head and each such valve is shunted by means of a bypass 26 which is simply a shunt conduit having an internal diameter of very reduced size compared with the internal diameter of the supply conduit to the end that only a restricted flow of etchant is able to reach spray head 20 through the bypass. This is of consequence, as will be made clear hereafter, only during intervals when the main valve with which any such bypass is associated is closed, temporarily to suspend etching in the particular station.

FIG. 2 further illustrates a preferred posture for mask 9 in an etching station. As there indicated, the mask has a dome-shaped portion 9a which is provided with a pattern of apertures having an initial diameter as required to accomplish phosphor screening of the tube in fabrication. The apertured portion 9a is circumscribed by a flange 9b and the mask is positioned with that flange directed vertically upward. The flange has the customary leaf springs for removably affixing the mask in operative relation to its screen and these springs facilitate removably coupling the mask to the conveyor frame 30 provided with coasters for riding along rails 31 to advance the masks from one etch station to the next.

Each etching station may be equipped with baffles that extend vertically downward from the ceiling of the station but leave a sufficient opening to accommodate the movement of masks or may have sliding doors which normally close the entrance and exit ports through which a mask may be moved into or may be moved from a particular station. In such a case, programmer 15 times or correlates the operation of the doors in relation to the step movement of conveyor 8.

It has been determined by experience that the past history of a mask has considerable bearing on its response to the etchant in the re-etch process and, therefore, it is highly desirable to provide a system for measuring the hole size of a mask after it has been etched in one of the stations to derive a control potential and to utilize that control potential to operate valves 25 of subsequent stations and adjust the etch time, if any, that occurs in those subsequent stations. More particularly and as described and claimed in Lerner application No. 6,619, a densitometer control 35 is associated with each of the etching stations. As indicated in FIG. 2 it comprises a light source 36 and the photocell 37 positioned on opposite sides of the path of travel of the mask and measures its hole size as a function of its transmissivity to light. Obviously, the larger the holes in the mask the more light emanating from source 36 is permitted to impinge photocell 37. The photocell 37 is coupled with a densitometer control 38 which essentially is a voltage comparison device and develops, as an output, a control voltage. The densitometer is calibrated to the end that its output voltage may control a subsequent stage and determine the amount of etching the mask is to receive in such sub sequent stage.

For greatest precision, all four etch stations are equipped with a densitometer 35 arranged so that the densitometer of the first, second and third stages control the duration of the etch time in the second, third and fourth stages, respectively. The densitometer of the fourth station is simply used as an indication or measure of the final aperture size of the re-etched mask. The control is easily attained by applying the output voltage of the densitometer of one station to energizing circuitry of the electrically operated main valve 25 of the next succeeding station. The system is preferably adjusted so that the mask does not attain its desired final hole size in the first etching station and therefore the densitometer associated with the first station permits valve 25 of the second etch station to remain open for all or at least part of the dwell time of that station. If the densitometer associated with the second station determines that the desired final aperture size has been attained, during the etch process of the second stage, it terminates the re-etch by closing valve 25 of the third station which interrupts the normal flow of etchant through that valve and therefore prevents etching from taking place in the third station. The same result is occasioned in the fourth stage when the densitometer associated with the third stage shall have made its measurement of hole size and shall have exercised its control over the fourth stage. This is all as described in the Lerner application.

During any operating interval in which a densitometer shall have closed one of valves 25, in the absence of the bypass 26, the flow of etchant to the station including that valve will be interrupted. Such an interruption has the possibility of introducing air blocks into the fluid distribution system and may also tend to introduce an undesirable cooling efiect on the etchant of some stage or stages relative to the others. Both of these effects are obviated in the described system because the bypass 26 for each valve 25 retains a flow of etchant into any station where the valve 25 has been closed but the flow through the bypass is so restricted in amount that the etchant emerging from nozzle array does not reach the height of the mask immediately above that spray head and, therefore, does not cause any etching of that mask. This is achieved by adjusting the distance between mask 9 within the etch station and the plane of rotation of the spray head. The distance is such that in the presence of etchant flow through valve the spray is uniformly distributed over mask 9 by the rotating spray head but when valve 25 is closed, the etchant continues to flow through the nozzle of the spray head although with insufficient force to reach mask 9.

The described arrangement has been found to improve the operation of the re-etch process. In particular, it avoids nonuniformities in the temperature of the etchant in the various etch stations and thereby permits more precision in re-etching which is highly desirable.

While a particular embodiment of the invention has been shown and described, modifications may be made, and it is intended in the appended claims to cover all such modifications as may fall within the true spirit and scope of the invention.

I claim:

1. A system for etching holes in shadow masks to a predetermined size comprising:

a plurality of etch stations, individually having spray means for directing an etchant vertically upward to contact and etch a mask instantaneously within the particular station;

means for conveying a mask through said stations, seriatim, with a dwell time of a given duration in each such station;

a fluid distribution system, including a source of etchant, for sustaining a flow of etchant between said source and said spray means of said station;

a like plurality of main valves in said distribution system individually in series with an assigned one of said spray means and a like plurality of bypasses individually shunted across an assigned one of said main valves for admitting, during intervals when its associated main valve is closed, a restricted flow of etchant to the spray means in series with said associated main valve of insufficient amount to etch a mask; and

a measuring system for measuring the hole size of a mask after it hasbeen etched in one of said stations to derive a control potential and for utilizing said control potential to selectively close said main valves and control etching of said mask in subsequent stations while retaining a restricted flow of etchant to the spray means of any of said stations in which said main valve has been closed.

2. A system in accordance with claim 1 in which,

said conveying means transports said mask through said stations at such a distance above said spray means that, during intervals in which said spray means receives etchant only through said bypass, said spray means is unable to spray said mask with said etchant.

3. A system in accordance with claim 2 in which,

said mask is essentially dome shaped and has a circumscribing flange;

and said conveying means transports said mask in a horizontal position with the flange thereof directed vertically upward;

References Cited UNITED STATES PATENTS 3,208,923 9/1965 Feiner et a1. 204-143 JACOB H. STEINBERG, Primary Examiner U.S. Cl. X.R.

ll88; 134-57 R, 72 

